Tanks are the rarest for that reason and others. Though I suppose one could claim that your states reason is the umbrella which all others fall. As tanking takes the most raid awareness, boss knowledge, gearing, and in cases of the upper-class NiM raiders; pure skill. All of them can be tossed under the umbrella of people just not wanting to more challenging things then needed. Though of course this is subject to a person's point of view. As not everyone is the same, some can tank and Dps the hardest content in he game but cannot heal a SM Op to save their life. Those kind of people would be the so-called exceptions to the rule (there is ALWAYS an exception to the rule lol).
As for the HK thing, chances are if the person who had aggro on him outranged him. It would force HK to move out and be attackable again. Presuming this is the same bug that I think it is.
My weird story;
Think I have not shared this yet. Did my last HM Cad ever (achievement hunter especially now with strongholds being tied to them), myself on Sin-tank, Sniper Dps, Merc Dps (I think might have been a second Sniper) and Sorc healer. We get to the elevator after the opening little area. Now I have already gone through my usual statements of;
"(Healer's name) yell at me if I am going to fast"
"I do hope to kill the bonus boss, as its my last achievement for here then I am 100%"
The healer just goes "g2g" and drops group. Now this healer was 23k-ish health, which I am not thrilled about on a healer. But I was getting to the point of asking him to prove he is heal spec. As I did not see anything that would tell me otherwise. The replacement ended up being a Merc healer but while we waited I was speaking with the Dps and learned that both were newish to the game. Which somewhat surprised me as they seemed knowledgeable of some stuff at least (one recognized my title for what it was, a NiM timed run one).
We go about our merry way with the new healer. And during both the Wookie and Gerald I swear he was DPS'ing. So I made the idle/joking comment of asking if he was bored to which he replied;
"Yeah.. Start taking damage so I got something to heal would ya?"
On the last boss I swapped out my helm, boots, and gloves to empty shells (have everyone warning that I was). And we down Ortol and afterwards the healer says;
"Now that was kinda fun, you actually took some damage"
We all had a good laugh and parted ways
I am sure I will have something from Gree event this week as I have like 5-10 toons I will be running it on. Need more basic comms! And grey helix components! And credits! ^.^